filter content for all site (e.g. by country)

Hello, is there or is it possible to have a limitation to filter the content, e.g. by a custom field ?

I mean

I plan to build a community for french speakers (france, belgium, luxembourg).

I would like to set different subdomains (be.mysite.org, fr.mysite.org, lu.mysite.org).

The main website (www.mysite.org) would show content from all countries; but if you go on one of the subdomain, it would filter the entire content (members, groups, messages…) to the selected country only.

We need a “global language filter” for all content, depending on what the user set as his preferred language in the wp-admin.

WordPress.com has such an language filter. You can choose “preferred language” and will be redirected to f.i. to de.wordpress.com (only content of people who set up their language and blog-language to german) or es.wordpress.com (only spanish).

Anyone also interested?

@grosbouff: Maybe we should add this feature request as an official ticket?